GPU: Do you really want to be going with a nVidia card right now? ATI is destroying them with the new 5xxx series. I highly recommend at least the HD 5870 for $400 (possibly the 5970 for $650ish if you can afford it).
PSU: You don't need 1000W, even for Crossfire/SLI. 750W would be more than enough for that. If you're buying it for Tri-fire (3 cards), don't. Using more than 2 cards doesn't scale up well, and is nothing but a waste of money.